Note: Electrospray ionisation is optimal for ionising liquid eluates from separation methods, as well as for the direct introduction of liquid samples to fragmentation in a mass spectrometer. Ionisation is made by positive or negative mode, depending on the ability of the molecules to form cations or anions. During the ionisation process, a biomacromolecule obtains a number of charges roughly proportional to its relative molecular mass. Also, non-covalent protein complexes may be ionised with ESI.